When Lucas finally decided to create the prequel trilogy back in 1994 I don't think he set out to make disappointing films. No filmmaker plans to make bad movies. 11 years later, we can look back at what might have gone differently had Lucas made some different decisions. (By the way, I think Ep III was a good film, not great but good).

Fateful choices Lucas made in making the prequels:

EPISODE I:
- forgoing his original late 70's vision for the prequel trilogy: According to original SW producer Gary Kurtz, EPISODE 1 was to focus on the origins of the Jedi Knights and how they are initiated and trained. EPISODE 2 would introduction and show the development of Obi-Wan Kenobi. EPISODE 3 was to introduce and show the life of Darth Vader. Lucas threw all that out the window in 94.
- deciding to begin the story with Anakin as a 9 year old boy. Originally he was going to be a teenager.
- choosing Jake Lloyd over Mason Gamble to play Anakin.
- casting Natalie Portman. An inexperienced child actress at the time who had no sci-fi or action film experience to play a pivotal role in the movies.
- deciding to start the trilogy with an entire episode about a dispute over taxation of trade routes on one small planet. Instead of diving right into the beginning of the Clone Wars.
- replacing Han Solo as the "comic relief" character in the film with a CGI creature named Jar Jar Binks.
- changing the force from an energy field to midi-chlorians
- saying Anakin built C3PO.
- Anakin's "virgin birth."
- going ahead with shooting/principal photography even though the script wasn't finished.
- not bringing in an outside screenwriter to help polish the screenplay. (He tried, but waited until filming began to contact "Empire/Jedi" scribe Lawrence Kasdan.
- not bringing in an outside director or at the very least someone to work with the actors on understanding the complicated plot and character motivation.
- not going with his initial idea for Ep I which would have emphasized story over action sequences. Originally the Episode wasn't going to end with a huge "Jedi" type battle montage.
- not brining in an outside editor to help assemble the film. Someone to fill the role left vacant by Marcia Lucas, Richard Chew and Paul Hirsch after "SW" and "ANH."

EPISODE II
- Again, beginning principal photography before the script was completed.
- using Jonathan Hales as a screenwriter instead of Lawrence Kasdan or Frank Darabont.
- forgoing his original vision for Episode II which was supposed to be about Obi-Wan Kenobi.
- Ignoring and removing most of Hales contributions to the updated script.
- Casting the extremely inexperienced (especially with big screen films) Hayden Christenson instead of Ryan Phillipe.
- Deciding to make Boba Fett's Dad the template for the stormtroopers and wasting the 2nd act of the movie explaining this.
- only showing one scene with Palpatine counseling Anakin.
- titling the movie "Attack of the Clones" instead of "The Clone Wars."
- again, not bringing in an outside director like Irvin Kershner. Or a Dialogue Coach to at the very least work with Natalie and Hayden on their romantic scenes.
- not using an outside editor again.

EPISODE III
- for the last time, going ahead with production even though the script wasn't completely finished.
- forgoing his original idea which was to start the film with a 7 planet montage showing the clone wars being fought.
- not having the space battle be more about the Federation fleet vs. the Republic cruisers.
- introducing the idea that Palpatine created Anakin by manipulating
midi-chlorians.
- putting the wookies in.
- having half of the movie focus on the capture of General Grievous.

I remember reading in the "Making of Ep III" book that when Lucas was on the set of the newly constructed Tantive IV he suggested they go back and re-film "A New Hope." It's too bad Lucas can't go back and refilm the first 2 episodes...